Prior art on thermal trimming of resistors has disclosed and addressed several configurations and variations. Thermal trimming of a thermally-mutable resistor is possible whether or not it is residing on or in a thermally-isolated microstructure. Also, there may or may not be a separate auxiliary heater used to apply heating power to a functional resistor which is intended to be trimmed. A sequence of electric voltage or current pulses is applied to a resistor (whether separate or not), in order to thermally trim the functional resistor.
For example, in Feldbaumer D W et al: “Pulse current trimming of polysilicon resistors”, simple functional resistors made from deposited polysilicon embedded within surface films in an integrated circuit chip can be thermally-trimmed by simply passing a high-enough current (or current density) through that resistor to sufficiently raise its temperature. Others propose a melting-segregation model to explain the self-trimming phenomena. U.S. Pat. No. 5,466,484 teaches the use of an auxiliary heating resistor to heat a functional resistor, where a heating voltage or current is applied to that separate heating resistor in order to raise the temperature of the functional resistor. WO 03/023794 discloses thermally-trimmable devices using thermally-isolated microstructures, such as micro-platforms suspended over cavities in silicon, in order to achieve higher temperatures at lower voltages and currents.
In general, thermal trimming has been disclosed, done by “self-trimming” where the electric trimming signals are applied directly to the functional resistor, or done by applying the trimming signals to a separate auxiliary heater-resistor. In both cases, there is potentially a fundamental problem with the stability of the resistor to which the trimming signals are applied. Hereafter in this text, this resistor will be referred to as the “heater-resistor”, regardless of whether it is the same physical resistor as the functional resistor targeted for thermal trimming. If this heater-resistor is itself thermally-mutable, then the temperature of the target functional resistor (which may be the same resistor as the heater-resistor) may change during the trimming signals, from one pulse to the next for a constant-level input signal, or even during the course of a single trimming pulse at a constant voltage or current. Such variations in electric resistance in the heater-resistor can lead to unpredictable changes in dissipated power, and loss of control over the trimming process. An example of this situation is depicted in FIG. 1.
Indeed, the heater-resistor typically is thermally-mutable. In the case of a separate auxiliary heater-resistor, it is often desirable to make the heater-resistor out of materials typically available in a standard semiconductor integrated circuit process—it is often not convenient to design or incorporate into the fabrication process a special material for the separate heater-resistor. For example, since the heater-resistor material must be compatible with trimming temperatures, in many processes the only practical material is a deposited polycrystalline silicon (or SiGe) resistor. Metals (e.g. Al, Cu) may melt at thermal trimming temperatures such as (700-800° C.), and resistors diffused into the bulk silicon cannot be effectively thermally isolated from the substrate well enough in order to trim resistors typically found sandwiched between surface dielectric layers. In the present state of the art, if the heater is manufactured from a non-thermally-trimmable material, or from a material which is relatively stable at the temperatures needed to thermally trim the functional resistor, then this may introduce extra complexity (expense, difficulty, and/or infeasibility), in the manufacturing process.